In the past, there have been cases in which a metal-oxide-semiconductor (MOS) transistor parasitic diode is used as a freewheeling diode in an inverter. In such cases, if a power supply voltage is applied in the reverse direction to the parasitic diode while an electrical current is flowing back in the forward direction to the parasitic diode, recovery current (reverse recovery current) flows to the parasitic diode, generating a large switching loss.
Japanese Unexamined Patent Application Publication No. H7-264876 discloses a method of preventing flow of recovery current by connecting a backflow prevention diode in the forward direction between a node on the high-voltage side and the drain of the MOS transistor, as well as connecting a freewheeling diode in the forward direction between the source of the MOS transistor and a node on the high-voltage side.
Also, Japanese Unexamined Patent Application Publication No. 2010-29019 discloses a method of preventing flow of recovery current by connecting two MOS transistors in series between a node on the high-voltage side and a node on the low-voltage side, and connecting a freewheeling diode having a high withstand voltage in the forward direction between a node on the low-voltage side and a node on the high-voltage side.
However, there is a problem with the method of Japanese Unexamined Patent Application Publication No. H7-264876, in that conduction loss is generated in the backflow prevention diode. There is also a problem with the method of Japanese Unexamined Patent Application Publication No. 2010-29019, in that costs become high as a result of providing two MOS transistors and a freewheeling diode having a high withstand voltage.
Accordingly, preferred embodiments of the present invention provide a highly efficient, low-cost switching power supply device, and an inverter, a converter, and a solar power controller including the same.
A switching power supply device according to a preferred embodiment of the present invention includes: a first transistor including a first electrode connected to a first node; and a second transistor including a first electrode connected to a second electrode of the first transistor, and a second electrode connected to a second node, wherein each of the first and second transistors includes a parasitic diode connected in a forward direction between the second and first electrodes, and a withstand voltage between the first and second electrodes of the first transistor is higher than a withstand voltage between the first and second electrodes of the second transistor. Additionally provided is a driving circuit that turns on the first and second transistors when a current flows from the first node to the second node, and turns on the first transistor while also turning off the second transistor when a current flows from the second node to the first node. The driving circuit turns on the first transistor by applying to a control electrode of the first transistor a voltage that is higher than a voltage obtained by adding a threshold voltage of the first transistor to a voltage of the second node. A first delay circuit that delays a potential variation of a control electrode of the first transistor and a second delay circuit that delays a potential variation of a control electrode of the second transistor are connected to the control electrode of the first transistor and the control electrode of the second transistor, respectively, and at least one of the first delay circuit and the second delay circuit includes a structure that causes a potential of the control electrode of the first transistor or the control electrode of the second transistor to be increased at a first speed and decreased at a second speed that is different from the first speed.
According to a preferred embodiment of the present invention, in a second transistor having a low withstand voltage compared to a first transistor having a high withstand voltage, a recovery current caused by a built-in diode is small. Subsequently, when current flows from a second node to a first node, the first transistor turns on and the second transistor turns off, and thus a current flows only to the built-in diode of the second transistor having a small recovery current, without a current flowing to the built-in diode of the first transistor having a large recovery current. Consequently, the built-in diodes of the transistors preferably are utilized without externally attaching a freewheeling diode, and losses due to recovery current are reduced. As a result, a highly efficient and low-cost switching power supply device is provided.
Also, according to a preferred embodiment of the present invention, the first transistor having a high withstand voltage and the second transistor having a low withstand voltage are connected in series, and the second transistor having a low withstand voltage performs the switching operation. Since the majority of the power supply voltage is applied between a first electrode and a second electrode of the first transistor having a high withstand voltage, the voltage between a first electrode and a second electrode of the second transistor having a low withstand voltage that performs the switching operation is able to be small. For this reason, the degree of influence on the electric potential of the control electrode of the second transistor that performs the switching operation exerted due to variation in the electric potential of the capacitively coupled first electrode is able to be small. As a result, misfiring of the second transistor due to variation in the electric potential of the control electrode is effectively reduced or prevented. Consequently, a highly reliable switching power supply device is provided.
Also, according to a preferred embodiment of the present invention, in at least one of either the first or the second transistor, it is possible to decide independently the speeds at which the electric potential of the control electrode increases and decreases. Consequently, since the rising and falling speeds of switching is able to be decided independently, more precise switching of the circuit is able to be sped up within a range in which the circuit does not oscillate, and losses are significantly reduced or eliminated.
The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings.